Many production processes maintain control of the temperature of individual units or elements within an overall system by refrigerating or heating the individual units during operation of the system. A particularly noteworthy and critical example of the type of demanding environment in which precise temperature control is needed is found in semiconductor fabricating processes. The manufacture of integrated circuits by forming multiple replicated patterns on semiconductor wafers involves numerous successive steps. Following image replication, intense energy concentrations are used to etch, deposit and otherwise treat successive layers on the wafer, but at the same time precise placement, alignment and dimensional stability of the wafer must be maintained during practically all such process steps. Furthermore the final product cannot accept even minuscule defects even though temperature differentials can tend to distort wafers, affect alignments and deteriorate pre-existing layers. Given these and other considerations, semiconductor fabrication facilities require vast capital expenditures to provide tools and support equipment meeting the conflicting demands of quality control and high volume output at the levels of resolution now demanded by the state of the art.
An example of one type of semiconductor fabrication equipment now in wide use is the cluster tool, in which different closely juxtaposed tools are used, singly or in combination, to transport, position, and complete different ones of a succession of processing steps quickly and efficiently. The tools within the cluster can vary widely in purpose and function. Some tools in the cluster may have to be refrigerated at times to levels as low as -40.degree. C., while at the same time others may have to be heated to levels as high as 80.degree. C. to 100.degree. C. The levels will vary during a process, but at any given time, the then chosen temperature level must be maintained closely at each operative tool. In addition, abrupt temperature changes are sometimes needed. For instance, extremely rapid cooldown of a tool at a transition point in a procedure may mean that the overall process can be significantly shortened or substantially more efficient. If transition times can be markedly shortened for tools when they are within an evacuated process chamber, the same chamber can be used again for a different step, without the need for returning the chamber to ambient pressure and reestablishing the vacuum, or utilizing a second chamber for the subsequent processing step.
The different tools in a cluster have heretofore largely been refrigerated or heated by individual units, each using a separate prime refrigeration source with a compressor/condenser system, or a separate heater. This not only affects reliability by increasing the number of critical and active operating units, but also requires that a substantial amount of floor space be dedicated to the cluster tool. However, every square foot of area required in a semiconductor fabrication facility is extremely costly. A large "footprint" size thus imposes a substantial economic penalty. Modern temperature control units for a cluster tool having, for example, six tools, require in the range of 18 square feet or more of floor area. Moreover, the service lifetime of these systems is limited because of the need to use multiple small refrigeration units, since they have shorter lifetimes than larger units and offer more chances of failure.
On-line maintenance of cluster tools requires that they be flushed of heat transfer fluid and disconnected from the temperature control unit. Current approaches typically use quick disconnects, which allow fluid to spill, and which tend to leak after a number of operations and impose a significant pressure drop in the system. An efficient subsystem for flushing and filling fluid used in temperature control is therefore highly desirable.
Precise control of the temperature of refrigerant-cooled fluid over a long service life is a desirable goal seldom achieved in practice. Solenoid valves, bimetallic proportional valves and other controls often used have inherent wear and hysteresis problems which affect both their accuracy and long-term life. Thermal expansion valves are capable of better resolution and proportional control, but present new problems when used in a refrigeration system for cluster tools, since it is the tool temperature which must be regulated, not superheat as in prior systems. In addition, prior thermal expansion valve systems are not usually able to effect extremely rapid cooldown because of thermal inertia problems.